This invention relates generally to a crystal oscillating reed and, in particular, to an AT-cut crystal oscillating reed.
Crystal oscillating reeds are well known in the art, as are the particular crystal oscillating reeds known as AT-cut crystal oscillating reeds. Crystal oscillating reeds are used in crystal oscillators. Clocks, computers, and other information communication equipment use crystal oscillators as reference frequency sources.
An example of an AT-cut crystal oscillating reed which is manufactured by a conventional etching method is disclosed in Japanese Patent Laid-Open No. 1-179513. This method of cutting an AT-cut crystal oscillating reed, shown in FIG. 1, is the most frequently used method for cutting a wide variety of crystal oscillating reeds. In FIG. 1, the X, Y and Z-axes denote the initial axial directions, whereas the Z' and Y'-axes denote the novel axial directions. The axial directions are obtained by rotating a parallel cut by an angular degree of .theta..degree..
A cross-sectional view of an AT-cut crystal oscillating reed which has been etched in accordance with the conventional method is shown in FIG. 2. Corrosion resisting films 122a and 122b and 123a and 123b are formed respectively on the upper and lower surfaces of a crystal wafer 121 to prevent unnecessary propagation of the etching. Crystal wafer 121 has a thickness of t. Upper corrosion resisting film 122 has a gap defined by upper ends 124 and 125. Similarly, lower corrosion resisting film 123 has a gap defined by lower at ends 126 and 127. The interval between ends 124 and 127 is shifted in the Z'-axis direction by t/tan .theta..degree.. Ends 125 and 126 are arranged to have a designated distance from ends 124 and 127, respectively, to maintain a width necessary to perform etching. In this conventional example, the distance between ends 124 and 125 as well as from ends 126 and 127 is arranged to be half the distance of the interval along the Z'-axis between ends 124 and 127, i.e., t/(2.times.tan .theta.).
Therefore, in the conventional method, the etching continues from both the upper and lower surfaces of crystal wafer 121 in the direction of the Z-axis until the two etching propagations encounter each other at a position where the thickness of crystal wafer 121 is t/2.
This conventional etching method for forming an AT-cut crystal oscillating reed has several disadvantages since the side surfaces (the surface between ends 124 and 126 of corrosion resisting films 122a and 123a, respectively, as well as between ends 125 and 127 of corrosion resisting films 122b and 123b, respectively) are disposed diagonally, i.e., not perpendicular to the corrosion resisting films. Additionally, the side surfaces are not smooth but have two "steps" and sharpened front portions. As a result, undesirable spurious oscillations can be generated, causing the oscillation characteristics to deteriorate. Moreover, the sharpened front portions can easily be broken, and over a long period of time the reliability of the reed will deteriorate. Finally, when this AT-cut crystal oscillating reed is clamped, cracks tend to form in the clamped portion. Thus, the clamped portion is easily broken and satisfactory oscillations cannot be generated.
A typical AT-cut crystal oscillating reed on which an electrode film has been formed is shown in FIGS. 3-5. Crystal oscillating reed 131 has two main surfaces 134 on the front and back thereof, as well as two side surfaces 135. Exciting electrodes 132 are formed on the main surfaces 134 of crystal oscillating reed 131. Electrodes 133, which are for establishing a connection with external equipment, are each formed on both main surfaces 134 and around side surfaces 135 so that the two main surfaces 134 of AT-cut crystal oscillating reed 131 are electrically connected to each other. Exciting electrodes 132, however, do not extend to the side surfaces 135. One of the external connection electrodes 133 is connected to each exciting electrode 132.
To form the electrodes, crystal oscillating reed 131 is positioned in a metal mask with apertures shaped in the form of the electrodes. Chrome and silver metals are then deposited on the metal mask and through the apertures to crystal oscillating reed 131.
When a voltage is applied to crystal oscillating reed 131 by external connection electrodes 133 which are connected to external equipment, an electric field is generated between exciting electrodes 132 so that crystal oscillating reed 131 is oscillated. Exciting electrodes 132 also act to confine the oscillation energy.
This conventional crystal oscillating reed 131 is less than completely satisfactory, however, because as the size of the oscillating reed is reduced, the crystal impedance (hereinafter, the "CI value") becomes excessively large. This occurs because the oscillation energy cannot be satisfactorily confined by the overlap effect of exciting electrodes 132.
Moreover, during electrode formation which utilizes the metal mask, satisfactory adhesion is difficult to obtain between the crystal oscillating reed and the mask. As a result, a clear contour and desired positional accuracy between crystal oscillating reed 131 and the metal mask is difficult to achieve, and so the crystal reed characteristics will not be uniform.
Accordingly, a method of etching an AT-cut crystal oscillating reed, an AT-cut crystal oscillating reed, and a method of forming electrodes on an AT-cut crystal oscillating reed which overcomes the problems of the conventional etching methods and reeds outlined above is desired.